RF inductors play an essential role in microwave front-end modules, including low noise amplifiers and voltage-controlled oscillators. For on-chip inductors, a spiral-shape inductor is often implemented. However, such implementation often exhibits a low quality factor Q at desired operating frequencies. Quality factor is the ratio of energy stored in the inductor to the energy loss in the inductor. Low-Q inductors drain more energy and generate excessive electronic noise. The lossy nature of silicon substrates employed with such inductors, as well as the resistive loss attributable to metal lines in such microelectronic devices, make it difficult to design high-Q inductors.
Substrate losses are at least partially caused by eddy currents induced by current flowing in the metal lines of the device. Approaches to minimize the eddy currents and, thereby, to reduce substrate losses include removing portions of the substrate and forming large doped regions encompassing the footprint of the inductor coil. However, the substrate removal approach results in poor resistance to environmental vibration and is difficult to integrate with existing fabrication processes. In addition, the large doped region approach provides little reduction in eddy current, partially because eddy currents can flow inside the doped regions.